Design Tolerances

What tolerances does everyone use when designing parts for printing on their reprap? i.e. If you have a 8mm bar, what size do you make the hole in the part to fit the bar?

While my entire parts have been coming out true to size, some of my holes seem to be undersized. I've seen this may be a problem with smaller holes, but I have some around 15mm that seem undersized. So I'm curious what tolerances everyone uses? Thanks!

Depends.
I usually use the actual size and I usually get a press fit, depending on how the hole is oriented during the print, I might ream the hole out to size.
NopHead has a good article on polyholes which is worth reading.
IME at least printing is not really conducive to high tolerances.

While outside dimensions are spot on, holes do tend to shrink. I use the values from metric.scad from the prusa git. Where m3=3.6mm m4=4.5mm and m8=9mm.

If you ream the holes, remember to add another perimeter or two when slicing, so you avoid breaking the walls.

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-Nudel
Blog with RepRap Comic

Thanks guys. Ive read Nopeheads work on holes. The metrics that you just mentioned Nudel seem spot on for me. I was thinking my calibration was screwed up since I needed to add .7-.8mm to my 8mm holes to get them to fit.